Apparatus and method for testing electrical systems having a plurality of terminals

ABSTRACT

Apparatus and method are provided for diagnostically testing electrical systems, such as printed circuit boards and integrated circuits. In testing electrical systems having a plurality of input terminals a test signal is applied to each input terminal and a comparison is made to verify whether or not any signal actually present on each input terminal corresponds with the applied test signal. In testing electrical systems having an output terminal which would normally provide a pulse signal in response to a test signal applied to an input terminal thereof, a test signal is applied to the input terminal and a signal comparison is made to verify whether any response occurring at the output terminal corresponds with a signal representative of the anticipated pulse signal response. Other features are disclosed including circuitry for selectively rendering various terminals of an electrical system as either input or output terminals, depending upon the types of tests to be made on the electrical system.

United States Patent [72] Inventors Jerome F. Walker;

Stuart F. Daniels, both of Shaker Heights, Ohio [21] Appl. No. 16,399 [22] Filed Mar. 4, 1970 [45] Patented Jan. 4, 1972 [73] Assignee Digital General Corporation Cleveland, Ohio [54] APPARATUS AND METHOD FOR TESTING ELECTRICAL SYSTEMS HAVING A PLURALITY 0F TERMINALS ll Clalms, 6 Drawing Figs.

[52] US. Cl 235/153, 324/73 R, 340/146.1 [51] Int. Cl C06! 11/00,

G01r 15/12 [50] Field oiSearch 340/1461; 235/153; 324/73 R [56] References Cited UNITED STATES PATENTS 3,049,692 8/1962 Hunt 340/ 146.1 N 3,096,511 7/1963 Taras 340/1461 X Primary Examiner-Charles E. Atkinson Attorney-Yount and Tarolli ABSTRACT: Apparatus and method are provided for diagnostically testing electrical systems, such as printed circuit boards and integrated circuits. In testing electrical systems having a plurality of input terminals a test signal is applied to each input tenninal and a comparison is made to verify whether or not any signal actually present on each input terminal corresponds with the applied test signal. In testing electrical systems having an output terminal which would normally provide a pulse signal in response to a test signal applied to an input terminal thereof, a test signal is applied to the input terminal and a signal comparison is made to verify whether any response occurring at the output terminal corresponds with a signal representative of the anticipated pulse signal response. Other features are disclosed including circuitry for selectively rendering various terminals of an electrical system as either input or output terminals, depending upon the types of tests to be made on the electrical system.

APPARATUS AND METHOD FOR TESTING ELECTRICAL SYSTEMS HAVING A PLURALITY OF TERMINALS This invention relates to the art of testing and, more particularly, to diagnostic testing of electrical systems.

The invention is particularly applicable for use in diagnostically testing printed circuit boards and integrated circuits; although it will be appreciated that the invention has broader applications and may be used, for example, in diagnostically testing various other types of electrical circuits.

In the production of electrical systems, such as printed circuit boards and integrated circuits, various complex manufacturing steps are involved. As the number of manufacturing steps and production volume increases quality control measures are frequently invoked. In addition to quality control of production steps and techniques, many manufacturers of electrical systems find it desirable to electrically test either ran domly selected production units or all production units to determine whether the units operate as designed.

Some electrical systems to be tested may include several input terminals. If test signals are applied to the various input terminals to determine whether a proper signal response is obtained, circuit malfunctions within the system may be undetected, particularly so if, in spite of such malfunctions, the proper signal response is obtained. A notable problem, for example, may exist where two or more input terminals of such a system are short circuited. Consequently, if electrical test signals of different levels or magnitudes are applied to the input terminals there will result a degradation in the level of the signals actually present at the input terminals. To adequately test such a system it is desirable to obtain verification as to whether the signals actually present on the input terminals correspond with the applied test signals.

Many electronic circuits used in digital or numerical control equipment generate or are operated by signal pulses. To adequately test such a system, it is desirable, in addition to input signal verification, to obtain verification as to whether any output pulse signal response obtained in response to a test input signal corresponds with the anticipated or unwanted pulse signal response.

To achieve these objectives it is also desirable that the test equipment employed include circuitry for each terminal of such a system for selectively rendering the terminal as either an input or an output terminal.

The present invention relates to apparatus and method for testing electrical systems to satisfy the foregoing objectives.

In accordance with one aspect of the present invention, apparatus and method are provided for testing an electrical system having a plurality of input terminals. This aspect of the invention provides that a test signal be applied to each input terminal, and a signal comparison be made to verify whether the test signal corresponds with any signal actually present at the input terminal.

Further in accordance with another aspect of the present invention, apparatus is provided for testing an electrical system having a plurality of terminals, and wherein for each terminal a binary signal source is employed to provide a test binary signal of a selected binary level. A switching means, such as a relay or a transistor, is interposed between the output of one binary signal source and one of the terminals so that in one switch condition the terminal serves as an input terminal to receive the test binary signal, and in a second switch condition the terminal serves as a system output terminal. Circuitry is also provided for each terminal to verify whether the binary level of any signal, either as in an input signal or an output signal, actually present at the terminal corresponds with the test binary signal.

The primary object of the present invention is to provide apparatus and method for testing electrical systems having a plurality of terminals in an efficient and economical manner.

Another object of the present invention is to provide apparatus and method for testing electrical systems wherein a signal comparison is made to verify whether the test input signals correspond with any signals which are actually present on the input terminals of such electrical systems.

Another object of the present invention is to provide apparatus for testing an electrical system having a plurality of terminals wherein circuitry is provided for selectively rendering the system terminals as either input or output terminals and wherein circuitry is also provided for verifying whether any signal actually present at such a terminal corresponds with a test signal for that terminal.

These and other objects and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention as read in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram illustration of one embodiment of the invention;

FIG. 2 is a combined schematic block diagram illustration of the circuitry employed in the embodiment of the invention shown in FIG. 1;

FIG. 3 is a combined schematic block diagram illustration of a binary comparator circuit which may be used in the embodiment of the invention shown in FIG. 1;

FIG. 4 is an illustration of an illustnative electrical system having a plurality of terminals and which is particularly applicable for being tested in accordance with the present invention;

FIG. 5 is a block diagram illustration of a second embodiment of the invention; and,

FIG. 6 is a combined schematic block diagram illustration of a circuit used in the second embodiment of the invention.

Referring now to the drawings, wherein the showings are for purposes of illustrating preferred embodiments of the invention only and not for purposes of limiting same, FIG. 1 illus trates one embodiment of the invention which generally includes a tape reader 20 for translating binary information stored in a programmed perforated paper tape 10; data directional gates 22 for receiving the binary information obtained from tape reader 20 and directing the information to an input-output register 24 and a level control register 26. The output of the level control register 26 is applied through a signal isolating amplifier 32 to an input-output switch 34 which is controlled by the input-output register 24. The output of the input-output switch 34 is applied to a tenninal 33 of an electrical system, or module, 36 under test. The input-output switch 34, under control of the inpu.t-output register 24, renders terminal 33 as either in input terminal of module 36 for receiving a test signal from register 26 through amplifier 32 and input-output switch 34, or renders the terminal 33 as the output terminal of module 36. Whether or not terminal 33 serves as an input terminal for receiving signals from register 26, or an output terminal for carrying any output signal from module 36, any signal actually present on terminal 33 is applied to one input of a comparator 42 for comparison with the test signal obtained from the output circuit of register 26. If the two signals do not correspond, then comparator 42 actuates an error indicator 44. Having briefly described the general operation of the invention, reference is now made to the following description of FIGS. 2 through 6 for a more detailed description of the preferred embodiments of the invention.

Reference is now made to FIG. 2, wherein there is illustrated an electrical system, or module, .36 having terminals 61, 62 and 63 to be tested in accordance with the present invention. For purposes of illustration, module 36' is shown as including an AND-gate 65. This AND-gate includes a pair of diodes 66, 67 having their anodes connected together in common and thence through a resistor 68 to a B+ voltage supply source. The cathodes of diodes 66 and 67 are respectively coupled to terminals 62 and 61. The common connected anodes of diodes 66 and 67 are also connected to terminal 63.

The diagnostic computer shown in FIG. 2 has a plurality of testing circuits A, B and C through N which are respectively used for testing an electrical system. As shown in FIG. 2, testing circuits A, B and C are respectively used in conjunction with terminals 63, 62 and 61 of module 36. The output circuits a, b, c of testing circuits A, B, C are coupled to terminals 63, 62 and 61, respectively, by means of a suitable electrical coupling 37.

The input-output register 24 includes a plurality of memory circuits which preferably take the form of transistorized flipflops 24a, 24b and 24c through 24n respectively included within the testing circuits A, B and C through N. Similarly, level control register 26 includes a plurality of memory elements in the form of transistorized flip-flops 26a, 26b and 26c through 26n respectively included within testing circuits A, B and C through N. These flip-flops may take any of the known forms of bistable memory elements and, hence, may be magnetic cores or ferroelectric capacitors. As shown, they include conventional transistorized flip-flop circuits each having a reset terminal R, a set tenninal S and an output terminal Q. Typically, in such a circuit, upon receipt of a reset signal at terminal R a binary or ground level signal appears at the output terminal Q. Thereafter, upon receipt of a binary l or positive signal at the set terminal S a binary 1" signal is present on the output terminal 0.

Each of the testing circuits A, B and C through N are identical and, hence, only circuit A will be described in detail herein with like components in the remaining testing circuits being identified with like character references followed by the designation a, b, c or n. Testing circuit A includes flip-flop 24a of the input-output register 24 having its output terminal Q coupled to ground through an operating coil 35a of a relay having normally open contacts 34a. Preferably, input-output switch 34 takes the form of encapsulated reed relay contacts 34a through 34n which are operated by operating coils, such as coil 35a, under the control of the associated input-output register flip-flops. Circuit A also includes a flip-flop 26a of the level control register 26 having its output terminal Q coupled to the input of a signal isolating amplifier 32a. The output of amplifier 32a is coupled through relay contacts 340 and test output circuit a to terminal 63 of module 36. Output terminal Q of flip-flop 26a is also coupled to one input of a binary comparator 42a. The output of comparator 42a is connected to the input of a light driver 46a, which may take the form, for example, of a simple transistor switch. The output circuit of light driver 46a is coupled to a lamp 45a. The combination of light driver 46a and lamp 450 define the error indicator 440 which has a corresponding equivalent in circuits B and C through N. A second input to the binary comparator 42a is taken from output terminal Q of flip-flop 52a constructed in the same fashion as flip-flops 24a and 26a. The set terminal S of flip-flop 52a is coupled directly to terminal 63 of module 36 through test output circuit a. The outputs of each binary comparator circuit 42a through 42!: are coupled to an OR- gate 48 and thence to a common alarm indicator 50. Alarm indicator 50 may, in the manner of individual error indicators 44a through 44 n, include a light driver together with an indicator lamp or buzzer. For purposes of testing the common alarm 50 and the individual error indicators 440 through 44n a test switch T is provided having inputs coupled to the common alarm 50 as well as to each of the light drivers 46a through 46n. Switch T, as shown in FIG. 2, is normally in an open condition, and when closed serves to apply a 13+ test signal through a resistor 51 to the inputs of each of the error indicators 44a through 44;: as well as the common alarm 50.

Reference is now made to FIG. 3 which illustrates one example of circuitry that may be employed for each binary comparator 42a through 42n. As shown in FIG. 3, with reference to binary comparator 42, there is provided a 'I'I'L NOR-gate 100, as well as an AND-gate 102, each having two inputs respectively taken from output terminal Q of flip-flop 26a and output terminal Q of flip-flop 52a. The output terminals of gates 100 and 102 are coupled to the input side of an OR-gate 104 which has its output coupled through a signal inverter amplifier 106 which, in turn, has its output coupled to the input of light driver 46a as well as to the OR-gate 48. The circuit shown in FIG. 3 will serve the function of providing a binary 0 signal at its output terminal so long as both input terminals receive binary signals of the same level; to wit, both input of gates 100 and 102 receive either binary 0" signals or binary l signals. In the event, however, one input receives a binary "0" signal and the other input receives a binary 1" signal then the output of inverter amplifier 106 will be a binary 1 signal for actuating both error indicator 440 as well as common alarm 50.

OPERATION OF FIRST EMBODIMENT For purposes of illustration, the inventive diagnostic computer may be used in conjunction with testing an AND-circuit 65 illustrated in FIG. 2 which has input tenninals 61 and 62 and an output terminal 63. It is contemplated that the AND circuit be tested by placing various combinations of binary l and binary 0 signals on terminals 61 and 62 and then observing output terminal 63 for a proper response. At least four tests should be considered for this AND gate; namely,

1. applying binary 1" signals to each of the terminals 61 and 62 and then checking terminal 63 for a positive or binary l signal;

2. applying a binary l signal to terminal 61, a binary 0" signal to terminal 62 and checking for a binary 0 signal at terminal 63;

3. applying a binary 0" signal to terminal 61, a binary l signal to terminal 62 and checking for a binary 0 signal at terminal 63; and,

4. applying binary 0 signals to terminals 61 and 62, and checking for a binary 0" signal at terminal 63. The foregoing tests would normally be considered different to fully check AND-circuit 65 for proper operation. However, if a circuit malfunction, such as a short circuit 70 shown by dotted lines in FIG. 2, exists within module 36 between terminals 61 and 62, then at least tests (2) and (3) above will not function properly since the actual signal levels at both input terminals will be at ground level or binary O signals. Similarly, test (1) will not function properly if either terminal 61 or 62 is shorted to ground within module 36. Also, test (4) will not function properly if, for example, an open circuit exists between the anodes of diodes 66 and 67 and the junction of terminal 63 and resistor 68. Consequently, in testing AND- circuit 65 for proper operation, it is desirable that verification be made as to whether any response existing at terminal 63 corresponds with the anticipated response, and whether the signals actually present at terminals 61 and 62 correspond with the test signals which presumably have been applied to these terminals.

The foregoing tests, as well as the input and output verifications, are obtained with the present invention. Referring to FIG. 2, each of the input-output register flip-flops 260 through 261: may be programmed to provide a binary 1" or binary 0" signal at its output terminal Q. For example, with respect to test circuit A, the binary level of the output signal appearing at output terminal Q of flip-flop 264 may be considered as a test signal for comparison with the actual signal existing on terminal 63 by means of binary comparator 42a. If the output signal of flip-flop 26a is also to be applied as an input signal to terminal 63, then input-output switch 340 must be closed. This function is obtained by programming flip-flop 24a so that when it carries a binary 1" signal at its output terminal 0 relay coil 35a is energized to close relay contacts 34a. Although flip-flops 24a and 26a may be manually controlled, as by pushbuttons directly connected to the reset terminals R and set terminals S, they are preferably controlled by obtaining information stored in a programmed memory, such as perforated paper tape 10. In programming the diagnostic computer the first step may be to apply a reset signal to each reset terminal R of each flip-flop within registers 24 and 26. This may be accomplished in several ways. One example may be utilizing a three channel, perforated tape, such as tape 10, driven passed tape reader 20 in the direction shown by arrow 12. For purposes of illustration, tape 10 is shown as having four operational steps; to wit, reset 1, in-out, reset 2 and test signals. For example, in the first step, reset 1, each channel is provided with a perforation shown as a 0 on tape 10 of FIG. 1. Each perforation, in accordance with standard programming techniques, is representative of a binary l signal. As tape 16 is driven in the direction of the arrow I2 and read by the tape reader 20, these three binary signals are applied through data directing gates 22 to the reset terminals of flip-flops 24a, 24b and 240 in the input-output register 24. This results in a binary signal being present at the output terminal Q of each of these three flip-flops.

In testing AND-gate 65, terminal 63 should serve as an output terminal, and terminals 62 and 61 should serve as input terminals. Consequently, in the second operational step programmed by tape 10, the three channels are provided with the code X00 respectively representative of a binary signal "0," a binary signal l and a binary signal l." This information is read by the tape reader 20 and the binary 0" signal is applied to set terminal S of flip-flop 24a and binary l signals are applied to set terminals S of flipflops 24b and 26c. Since the output terminal 0 of flip-flop 24a will now carry a binary "0" signal, operating coil 35a will not be energized and, hence, relay contacts 34a will remain open so that terminal 63 serves as an output terminal. The output terminals 0 of flip-flops 24b and 240 carry binary l signals and, consequently, relay contacts 34b and 340 will be closed so that terminals 62 and 61 respectively serve as input terminals to AND-gate 63. The third step in the sequence may be considered as reset 2, wherein tape is programmed with three apertures, 000, representative of three binary l signals. Once this information is read by tape reader 20, binary l "signals are applied to reset terminals R of each flip-flop 26a, 26b and 26c so that the output terminals Q of these three flip-flops carry binary 0" signals. The fourth step in this sequence of operation may be considered as the test signal operation. This operation is shown in FIG. I as a program wherein binary 0" signals are to be applied to set terminals S of flip-flops 26a and 26b and a binary 1 signal is to be applied to set terminal S of flip-flop 26c. This will be recognized as corresponding with test (2) discussed hereinbefore.

In accordance with test (2) binary l signal is applied from the output terminal Q of flip-flop 260 through isolating signal amplifier 32c, and the now closed relay contacts 36c to input terminal 61 of AND-gate 65. Similarly, a binary "0 signal is applied from the output tenninal Q of flip-flop 26b through isolating signal amplifier 32b, now closed relay contacts 34b to the input terminal 62 of AND-gate 65. The proper response to these input signals should be a binary 0" signal at output terminal 63 of the AND-gate. Since terminal 63 is connected to the set tenninal S of flip-flop 52a, the output terminal 0 of this flip-flop will now carry a binary 0 signal. The test signal taken from the output terminal Q of flip-flop 26a is a binary 0" signal. Thus, both inputs to binary comparator 62a are binary 0 signals and, hence, a correct response has been obtained. If, for some reason, the binary level of any signal actually present at output terminal 63 is a binary l signal, then flip-flop 52a would have a binary l signal at its output ter' minal Q. Binary comparator 4211 would then provide a binary 1" signal at its output terminal for application to the light driver 46a and, hence, energize indicator lamp 65a. At the same time, the output circuit of binary comparator 62a would also provide a binary l signal through OR-gate 63 to energize the common alarm 50.

If, during the foregoing testing operation, a short circuit 76 is present within module 36', then the binary 1" signal obtained from flip'flop 26c would be degraded at terminal 611 to a ground potential, or binary 0" signal. This binary 0" signal is coupled to the set input S of flip-flop 52c so that the output terminal Q of this flip-flop now carries a binary 0" signal. However, the test signal obtained from output terminal 0 of level control register flip-flop 260 is a binary 1 signal and, hence, binary comparator 420 will provide a binary l signal at its output terminal for energizing error indicator 44c as well as common alarm 50. By the foregoing operation, each circuit B and C serves to verify whether the intended input signal corresponds with the level of the signal actually present on the input terminals of module 36, and, in addition, circuit B verilies whether or not any output signal obtained at output terminal 63 corresponds with the anticipated output signal.

PULSE RESPONSE VERIFICATION In the description of operation thus far circuits A, B and C of FIG. 2 have been described in conjunction with the operation obtained with steady state signals being applied to terminals 61 and 62, or obtained from terminal 63 of module 36'. In these steady state operations, flip-flops 32a, 52b and 62c may be dispensed with. They are, however, included within the circuitry for purposes of pulse response verification. For example, FIG. 6 illustrates an electrical system, or module 36", which while having terminals 61, 62' and 63' corresponding with terminals 61, 62 and 63 of module 36', includes a substantially different electrical circuit 70. Circuit 76 is provided as an example of an electrical system which has an output terminal 63 for providing a pulse signal in response to a test signal applied to an input terminal 62', This circuit may take various forms, such as, for example, a monostable oscillator 72 having its input connected through a capacitor 74 to terminal 62' as well as through a resistor 76 to terminal 61 The output circuit of oscillator 72 is coupled to output terminal 63. As is well known, a monostable oscillator, such as oscillator 72, will, upon receipt of an input signal, provide at its output terminal a pulse response of a given polarity and magnitude for a specific period of time. For purposes of testing circuit 70, output circuits a, b and c of test circuits A, B and C of the circuitry shown in FIG. 2 may be respectively connected, as by an electrical coupling 37, to terminals 63, 62' and 611'. In programming the diagnostic computer for testing circuit 70, circuits A, B and C may be operated so that terminal 61 receives a binary "0 or ground signal, terminal 62 serves as an input terminal, and terminal 63 serves as an output terminal. With all of the flip-flops being reset, then flipflop 26b may be actuated to provide a test input signal, such as a binary l signal. This will be applied through coupling capacitor 76 to actuate the oscillator 72 so that, if the oscillator circuit operates properly, an output signal such as a positive polarity signal of a given magnitude and duration will appear at output terminal 63'. For purposes of verifying whether the correct output signal is obtained at terminal 63, flip-flop 26a should be programmed so that its output terminal 0 carries a binary 1" signal. If, as is anticipated, output terminal 63' provides a momentary pulse response to the input signal applied to terminal 62, and if the pulse response is a positive or binary 1" signal pulse flip-flop 52a will provide a binary l signal at its output terminal Q. Binary signal comparator 42a serves to verify that the pulse signal] response at terminal 63 is of the same binary level as the binary signal provided at the output terminal 0 of flip-flop 26a. If, as anticipated, both signals are binary l signals, then no error indication is made. If, on the other hand, no signal response is noted at output terminal 63, or if the signal response is a negative pulse signal, then the output terminal Q of flip-flop 52a will carry a binary 0 signal, whereupon the binary signal comparator 420 will energize error indicator Ma as well as the common alarm 50.

SECOND EMBODIMENT Referring now to FIGS. 5 and 6, there is shown a second embodiment of the invention. As will be noted, the block diagram shown in FIG. 5 is quite similar to that as shown in FIG. 11, and for purposes of simplifying the description of the invention like components are identified with like character references. Also, for purposes of simplifying the description of the invention only those components which have been changed or added will be discussed in greater detail below.

In the description of operation with respect to testing circuit 70, shown in FIG. il, reference was made to signal verification of a positive pulse response at output terminal 63. In the event the anticipated pulse response :is a negative pulse response, then if the test signal is a binary 1" signal the pulse response from terminal 63 should be inverted to obtain signal verification. To provide flexibility of choice in testing circuit 70 for either a negative or positive pulse response, each test circuit A, B and C through N shown in FIG. 2 may be modified as shown in FIG. 6 for circuit A' corresponding with circuit A of FIG. 2. Circuit A corresponds with circuit A with the exception of the addition of gated signal inverter circuit 540, interposed between output circuit a and the set terminal S of flip-flop 52a, and an inverter register flip-flop 56a, which is constructed in the same fashion as flip-flops 24a and 260. As shown in FIG. 6, the output terminal Q of flip-flop 56a is coupled to one input of an AND-gate 1 10 having its second input coupled to the output of a signal inverter amplifier 112. The input circuit of the signal inverter 112 is, in turn, coupled to output circuit a of test circuit A. The output of AND-gate 110 is coupled to one input of an OR-gate 114 having a second input coupled to the output of an AND-gate 113 having one input coupled to output circuit a of test circuit A. The second input to AND-gate 113 is taken from the output of a signal inverter amplifier 115 having its input coupled to output terminal Q of flip-flop 56a. The output of OR-gate l 14 is coupled to the set terminal S of flip-flop 520.

During the testing operation, each of the testing circuits which are constructed as circuit A may be selectively actuated to verify either a positive or negative pulse signal response from output terminal 63 of circuit 70. In the event verification is to be made with respect to a positive pulse signal response from circuit 70, then inverter register 560 will be programmed so that its output terminal Q carries a binary 0" signal and flip-flop 26a is programmed so that its output terminal Q carries a binary l signal. In the event that the actual signal response at output tenninal 63' is a positive signal pulse, i.e., a binary l signal, then this signal will be inverted by signal inverter amplifier 112 to appear as a l signal to the input circuit of AND-gate 110. Since the second input of AND-gate 110 receives a binary 0" signal, the output of AND-gate 110 will not carry a binary l signal. However, the second input to OR-gate 114 will receive the momentary binary l pulse response from output terminal 63' through AND-gate 113 and this will be applied to the set terminal of flip-flop 52a. Consequently, both inputs to the binary signal comparator 42a will be binary "1" signals and signal verification has been achieved. If the response actually present on output terminal 63 is either a ground potential or a negative potential, then the second input to AND-gate 110 will be a positive 1" signal and the output of AND-gate 110 will remain unchanged. With such a signal being noted at output terminal 63', flip-flop 52a will provide a binary "0" signal at its output terminal, and comparator 42a will actuate the error indicator 44a.

In the event the anticipated pulse signal response from output terminal 63 is a negative signal, then flip-flop 56a will be programmed to provide a binary 1 signal at its output terminal Q, and flip-flop 26a will be programmed to provide a binary l signal at its output terminal Q. If, as is anticipated, a momentary negative pulse appears at output terminal 63 this signal will be converted by signal inverter amplifier 112 to a binary 1" signal for application to one input of AND-gate 110. Since the second input to this AND-gate is also a binary l signal, a binary 1" signal will be applied to the set terminal of fiip-flop 52a. Thus, both inputs to the binary signal comparator 420 will be binary l signals and, hence, signal verification has been accomplished, and no error indication will be present. If any output signal response obtained at output terminal 63 is either a ground or positive signal, then the output terminal Q of flip-flop 52a will apply a binary 0" signal to binary signal comparator 42a. Consequently, since a disparity exists in the binary levels of the two input signals to comparator 42a, the error indicator 44a will be energized.

With the modification of FIG. 6 made to each of the test circuits A, B and C through N of FIG. 2, the block diagram of the modified diagnostic computer will appear as shown in FIG. 5. As will be noted, this block diagram differs from that of FIG. 1

only by the inclusion of signal inverter register 56, a gated signal inverter 54 together with blocks and 122 respectively representative of reset and test operations. The inverter register 56 preferably includes a flip-flop for each test circuit A, B and C through N, such as flip-flop 560 shown in FIG. 6. Similarly, the gated signal inverter 54 preferably includes for each test circuit A, B and C through N a signal inverter such as circuit 54a shown in FIG. 6. The reset function may be accomplished with the use of a reset register 120 having a single flipflop circuit, constructed in the same manner as flip-flop 24a within register 24. This flip-flop, when programmed, should receive a binary 1 signal from the tape reader and data directing gates to, in turn, actuate the reset terminal R of all flip-flops within the various registers of the diagnostic computer prior to commencing a testing operation. Similarly, the test function, as shown by block 122, may be accomplished with a register having a single flip-flop corresponding with, for example, flip-flop 24a, and which should be programmed to receive a binary "1" signal from the tape reader and data directing gates for purposes of performing the same function as switch T shown in FIG. 2; to wit, of applying binary 1" signals for energizing error indicators 44a through 44n as well as a common alarm 50.

Although the invention has been shown and described in connection with preferred method steps and apparatus embodiments, it will be readily apparent to those skilled in the art that various changes in method steps and in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention as defined by the appended claims.

Having thus described our invention we claim:

1. Apparatus for testing an electrical system having a plurality of input terminals and comprising a test circuit for each said input terminal, each said test circuit including:

a test output circuit adapted to be electrically coupled to a said input terminal;

means for applying a test signal to said test output circuit and, hence, to said input tenninal; and

input verification means coupled to said test output circuit and, hence, to said input terminal for receiving both said test signal and any signal actually present at said input terminal for electrically verifying whether said test signal corresponds with any signal actually present at said input terminal.

2. Apparatus as set forth in claim I, wherein said test signal applying means includes binary signal means for selectively applying a binary 1" or binary 0" test signal to said input tenninal and said input verification means serves to verify whether the binary level of said test signal corresponds with the binary level of any signal actually present at said input terminal.

3. Apparatus as set forth in claim 2, wherein said input verification means includes binary signal comparator means for providing an error output signal when the binary levels of said test signal provided by said binary signal means and any signal actually present at said input terminal do not correspond.

4. Apparatus as set forth in claim 3, including signal isolation means interposed between the output of said binary signal means and said test output circuit, said signal comparator means having one input for receiving said test signal from a circuit point between the output of said binary signal means and said signal isolation means and a second input coupled to said test output circuit for receiving any signal actually present at said input terminal.

5. Apparatus for testing an electrical system having a plurality of terminals and comprising for each said terminal:

binary signal means for providing a test binary signal of a selected binary level;

actuatable switching means adapted to be interposed between the output circuit of said binary signal means and a said terminal, said switching means having a first condition wherein a circuit is completed between the output circuit of said binary signal means and said terminal so that said terminal serves as an input terminal for receiving said test binary signal and a second condition wherein an open circuit is obtained between the output circuit of said binary signal means and said tenninal so that said terminal serves as an output terminal; and

signal verification means for receiving a said test binary signal and any signal actually present at said terminal for verifying whether the binary level of any signal actually present at said terminal corresponds with that of said test binary signal.

6. Apparatus as set forth in claim 5, wherein said signal verification means includes binary signal comparator means for providing an output error signal when the binary level of any signal actually present at said terminal does not correspond with that of said test binary signal.

7. Apparatus as set forth in claim 6, including isolation means interposed between the output of said binary signal means and said terminal for, when said terminal serves as an input terminal for receiving said test binary signal, preventing the binary level of said test signal appearing at the output circuit of said binary signal means from being degraded as a result of a circuit malfunction within said electrical system being tested, said binary signal comparator means having one input for receiving said test binary signal from a circuit point between the output of said binary signal means and said isolation means and a second input for receiving any signal actually present at said terminal.

8. Apparatus as set forth in claim 5, wherein said electrical system being tested has a first terminal for providing a pulse signal in response to an input signal applied to a second terminal;

control means for actuating said switching means for said first terminal to said first condition so that said first terminal serves as an output terminal and for actuating said switching means for said second terminal to said second condition so that said second terminal serves as an input terminal, whereby said input tenminal receives said test binary signal from the binary signal means for said input terminal as an input signal and said signal verification means for said first terminal serves to verify whether the binary level of any signal actually present at said output terminal corresponds with said test binary signal from the binary signal means for said output terminal.

9. Apparatus as set forth in claim 8, including bistable means having an input circuit for receiving any actual signal present at said output terminal and an output circuit for providing a response binary signal when said actual signal is a pulse signal of a given polarity; and

said signal verification means includes binary comparator means for providing an output error signal when the binary levels of said response binary signal and said representative binary signal do not correspond. 10. Apparatus as set forth in claim 9, including pulse signal inverting means interposed between said output terminal and said bistable means for inverting the polarity of any said actual pulse signal at said output terminal so that if said actual pulse signal is of the wrong polarity it will be presented to said binary signal comparator means as being of said given polarity.

ill. A method for testing electrical systems having a plurality of input terminals and comprising the steps of:

utilizing binary signal generating means for selectively providing for each input terminal a test binary 1" or test binary 0 signal for application to said input terminal;

for each said input terminal, utilizing; a selected test circuit for applying a said selected binary signal to said input terminal; and

for each said input terminal, utilizing said selected test circuit having signal verification means for providing an error indication when said test binary signal for said input terminal is of a different binary level from that of any signal actually present at said input terminal. 

1. Apparatus for testing an electrical system having a plurality of input terminals and comprising a test circuit for each said input terminal, each said test circuit including: a test output circuit adapted to be electrically coupled to a said input terminal; means for applying a test signal to said test output circuit and, hence, to said input terminal; and input verification means coupled to said test output circuit and, hence, to said input terminal for receiving both said test signal and any signal actually present at said input terminal for electrically verifying whether said test signal corresponds with any signal actually present at said input terminal.
 2. Apparatus as set forth in claim 1, wherein said test signal applying means includes binary signal means for selectively applying a binary ''''1'''' or binary ''''0'''' test signal to said input terminal and said input verification means serves to verify whether the binary level of said test signal corresponds with the binary level of any signal actually present at said input terminal.
 3. Apparatus as set forth in claim 2, wherein said input verification means includes binary signal comparator means for providing an error output signal when the binary levels of said test signal provided by said binary signal means and any signal actually present at said input terminal do not correspond.
 4. Apparatus as set forth in claim 3, including signal isolation means interposed between the output of said binary signal means and said test output circuit, said signal comparator means having one input for receiving said test signal from a circuit point between the output of said binary signal means and said signal isolation means and a second input coupled to said test output circuit for receiving any signal actually present at said input terminal.
 5. Apparatus for testing an electrical system having a plurality of terminals and comprising for each said terminal: binary signal means for providing a test binary signal of a selected binary level; actuatable switching means adapted to be interposed between the output circuit of said binary signal means and a said terminal, said switching means having a first condition wherein a circuit is completed between the output circuit of said binary signal means and said terminal so that said terminal serves as an input terminal for receiving said test binary signal and a second condition wherein an open circuit is obtained between the output circuit of said binary signal means and said terminal so that said terminal serves as an output terminal; and signal verification means for receiving a said test binary signal and any signal actually present at said terminal for verifying whether the binary level of any signal actually present at said terminal corresponds with that of said test binary signal.
 6. Apparatus as set forth in claim 5, wherein said signal verification means includes binary signal comparator means for providing an output error signal when the binary level of any signal actually present at said terminal does not correspond with that of said test binary signal.
 7. Apparatus as set forth in claim 6, including isolation means interposed between the output of said binary signal means and said terminal for, when said terminal serves as an input terminal for receiving said test binary signal, preventing the binary level of said test signal appearing at the output circuit of said binary signal means from being degraded as a result of a circuit malfunction within said electrical system being tested, said binary signal comparator means having one input for receiving said test binary signal from a circuit point between the output of said binary signal means and said isolation means and a seconD input for receiving any signal actually present at said terminal.
 8. Apparatus as set forth in claim 5, wherein said electrical system being tested has a first terminal for providing a pulse signal in response to an input signal applied to a second terminal; control means for actuating said switching means for said first terminal to said first condition so that said first terminal serves as an output terminal and for actuating said switching means for said second terminal to said second condition so that said second terminal serves as an input terminal, whereby said input terminal receives said test binary signal from the binary signal means for said input terminal as an input signal and said signal verification means for said first terminal serves to verify whether the binary level of any signal actually present at said output terminal corresponds with said test binary signal from the binary signal means for said output terminal.
 9. Apparatus as set forth in claim 8, including bistable means having an input circuit for receiving any actual signal present at said output terminal and an output circuit for providing a response binary signal when said actual signal is a pulse signal of a given polarity; and said signal verification means includes binary comparator means for providing an output error signal when the binary levels of said response binary signal and said representative binary signal do not correspond.
 10. Apparatus as set forth in claim 9, including pulse signal inverting means interposed between said output terminal and said bistable means for inverting the polarity of any said actual pulse signal at said output terminal so that if said actual pulse signal is of the wrong polarity it will be presented to said binary signal comparator means as being of said given polarity.
 11. A method for testing electrical systems having a plurality of input terminals and comprising the steps of: utilizing binary signal generating means for selectively providing for each input terminal a test binary ''''1'''' or test binary ''''0'''' signal for application to said input terminal; for each said input terminal, utilizing a selected test circuit for applying a said selected binary signal to said input terminal; and for each said input terminal, utilizing said selected test circuit having signal verification means for providing an error indication when said test binary signal for said input terminal is of a different binary level from that of any signal actually present at said input terminal. 